1. Field of the Invention
The present invention relates to vascular prostheses improved through the use of an external sheath that is resistant to cell ingrowth while being permeable to flux of fluid and macromolecules across the thickness of the sheath. The sheath can also impart mechanical strength to the vascular tissue of the prostheses.
2. Description of Related Art
Blood vessels taken from human or animal donors have been widely used to replace blocked, aneurysmal, or otherwise damaged arteries and veins. Both living and preserved human arterial allografts were extensively used as arterial substitutes in the 1950's and this initial use appeared successful. However, human allografts were largely abandoned by 1960 due to a high incidence of thrombosis, stenosis, and aneurysmal dilatation.
The practice of grafting with allogeneic arteries was abandoned in favor of the mechanically superior alternative of using vascular grafts made of synthetic materials. The synthetic materials principally used for this grafting are polyethylene terephthalate and expanded polytetrafluoroethylene (ePTFE). These materials offer biocompatibility and provide sufficient mechanical integrity to prevent aneurysmal dilation. Prosthetics made of these materials were shown to be successful in the replacement of large vessels such as the aorta or iliac arteries and are still successfully used in these applications. Although successful when used in large diameter applications, the patency performance of these materials in demanding small diameter applications such as coronary artery bypass or peripheral arterial bypass distal to the popliteal artery, for example, has been substantially less than that of transplanted living autologous vessels.
Regardless of whether a prosthetic vascular graft is obtained from a donor or is made of synthetic materials, it has been a common belief since the 1950's that ingrowth of host tissue into vascular grafts leads to improved function. Representative of this belief is the following statement: "With existing prostheses, we believe that the best assurance of long term patency without complication is attainment of complete healing of the prosthetic wall, which includes an endothelialized flow surface. This final healed state is principally dependent upon the ingrowth of areolar tissue from perigraff sources through the interstices of a pervious graff wall." (Sauvage et al., "Future Directions in the Development of Arterial Prostheses for Small and Medium Caliber Arteries," Surgical Clinics of North America 54:213-228 (1974))
With biologically-derived prosthetics, host tissue ingrowth is observed to alter the donor graft material into one composed at least partially of recipient tissues. In addition, such ingrowth of autologous tissues usually generates a blood interface in the prosthetic replacement composed in part of tissues of the host.
In the case of porous synthetic materials, autologous tissue ingrowth occurs through the interstices of the material. The apparent desirability of ingrowth of autologous tissue into grafts made of porous polytetrafluoroethylene (ePTFE) is illustrated by the results of a study comparing the patency performance of small caliber vascular ePTFE prosthetics having different porosities. The study showed that high porosity ePTFE grafts with a fibril length of 100 .mu.m had pronounced ingrowth of fibrous tissue into the interstices of the graft and remained 100% patent at six months postoperatively. Low porosity ePTFE grafts with a short fibril length (10 ) had layers of tissue on either side of the graft wall, but no ingrowth of tissue into the interstices of the graft. These low porosity grafts were 67% patent at six months postoperatively. (Florian et al., "Small Vessel Replacement With Gore-tex (Expanded Polytetrafluoroethylene)," Arch. Surg. 111:267-270 (1976))
The initial theory concerning performance of vascular grafts was proposed by Wesolowski in 1963. This theory, based upon the conclusion that ingrowth of host cells leads to better patency performance, remains the predominate theory today. (See for example, Stanley et al., "Biologic and Synthetic Vascular Grafts," Vascular Surgery, A Comprehensive Review, Fourth Edition, Chapter 21, pp 370-389, .COPYRGT.1993, referring to Wesolowski, S. A., Dennis, C., "Fundamentals of Vascular Grafting," New York, McGraw-Hill, 1963)
Intentional exclusion of host cells from the interstices and lumen of a vascular graft in order to prevent the host cells from adversely affecting the patency performance of the vascular graft has not been previously described. Moreover, application of an external porous synthetic sheathing material to a vascular graft that prevents disruption or remodeling of the graff by host cells while being permeable to the flux of biological fluids and molecules across its thickness has heretofore not been described.